As an example of a mobile communication system to which the present invention is applicable, a 3rd generation partnership project long term evolution (3GPP LTE) (hereinafter, referred to as LTE) communication system is described in brief.
FIG. 1 is a diagram schematically illustrating a network structure of an E-UMTS as an exemplary wireless communication system.
An evolved universal mobile telecommunications system (E-UMTS) is an advanced version of a legacy universal mobile telecommunications system (UMTS) and basic standardization thereof is currently underway in the 3GPP. E-UMTS may be referred to as an LTE system. For details of the technical specifications of UMTS and E-UMTS, see Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.
Referring to FIG. 1, E-UMTS includes a user equipment (UE), evolved Node Bs (eNode Bs or eNBs), and an access gateway (AG) which is located at an end of an evolved UMTS terrestrial radio access network (E-UTRAN) and connected to an external network. The eNBs may simultaneously transmit multiple data streams for a broadcast service, a multicast service, and/or a unicast service.
The UE is fixed or mobile. The UE is a device that transmits and receives user data and/or various kinds of control information though communication with a base station (BS). The term ‘UE’ may be replaced with ‘terminal equipment’, ‘Mobile Station (MS)’, ‘Mobile Terminal (MT)’, ‘User Terminal (UT)’, ‘ Subscriber Station (SS)’, ‘wireless device’, ‘Personal Digital Assistant (PDA)’, ‘ wireless modem’, ‘handheld device’, etc. A BS is typically a fixed station that communicates with a UE and/or another BS. The BS exchanges data and control information with a UE and another BS. The term BS' may be replaced with ‘Advanced Base Station (ABS)’, ‘Node B’, ‘evolved-Node B (eNB)’, ‘Base Transceiver System (BTS)’, ‘Access Point (AP)’, ‘Processing Server (PS)’, etc. In the following description, BS is commonly called eNB.
One or more cells are managed by one eNB. A cell is configured to use one of bandwidths of 1.25, 2.5, 5, 10, 15, and 20 MHz to provide a downlink (DL) or uplink (UL) transmission service to multiple UEs. Different cells may be configured to provide different bandwidths. The eNB controls data transmission and reception to and from a plurality of UEs. For DL data, the eNB transmits DL scheduling information to notify a corresponding UE of time/frequency resources through which the data is to be transmitted, coding, data size, and hybrid automatic repeat and request (HARQ)-related information. For UL data, the eNB transmits UL scheduling information to a corresponding UE to inform the UE of available time/frequency resources, coding, data size, and HARQ-related information. An interface for transmitting user traffic or control traffic between eNBs may be used. A core network (CN) may include an AG and a network node for user registration for the UE. The AG manages mobility of the UE on a tracking area (TA) basis, each TA including a plurality of cells.
Conventionally, the legacy LTE communication scheme mainly considers wireless communication between an eNB and a UE. However, demands for technology enabling direct communication between UEs have recently increased.
FIG. 2 is a conceptual diagram illustrating direct communication between UEs.
Referring to FIG. 2, UE-to-UE direct communication is performed between UE1 and UE2 and between UE3 and UE4. The eNB may control positions of the time/frequency resources, transmit power and the like for direct communication between UEs through a proper control signal. Direct communication between UEs is referred to as device-to-device (D2D) communication in the following description.
D2D communication has different requirements from legacy LTE communication in many aspects.